The present invention relates to a signal sensing system for production wells and notably for wells producing petroleum effluents, which allows the mechanical uncoupling of the sensors. The reception system according to the invention is particularly suitable for carrying out measurings in wells where progressing is difficult either because of the more or less strong inclination of the well in relation to the vertical, or because of its narrowness.
Such a signal sensing system has numerous applications. It can be used to detect the vibrations generated on the audio frequency band by the rocks in geologic formations when they are subjected to hydraulic fracturings, during the pumping periods which follow these fracturings or during a long pumping period in injection or else during the production period of a well. This sonde can be utilized for seismic prospecting operations implying the positioning of sensors in one or several wells. Such a signal sensing system may also have other applications such as in the monitoring of formations, of structures or of storage cavities, etc, or else for detecting seismic activities or earthquakes. More generally, the reception system which is the object of the invention can be used in all concerned activities where acoustic signals are to be picked up in a frequency band up to several thousand Hertz.
A well equipped for producing petroleum generally comprises casing, which is taken down into a borehole and immovably attached to the walls by injecting cement into the annular space between it and the well. In the production zone of the well, the casing is fitted with lateral perforations communicating with the formations surrounding the well. A flow tubing string with a section generally much smaller than that of the casing is taken down into the latter. A sealing device is arranged at the base of the string and closes the annular space between it and the casing.
Carrying out measurings in the production zone can be done by means of a standard tool with a relatively large section providing that the tubing string is first taken up, but this solution is often dismissed because of the difficulties and the delays it causes.
The subterranean production zone is generally reached by taking down into the equipped well a sonde or a tool with a section sufficiently small to pass within the flow string and go through the section restrictions due to various sealing devices (tubing packers or pump seats for example). Sondes implementing this solution are described for example in the following patents: U.S. Pat. Nos. 4,744,416, 4,737,636 4,737,636 or 4,724,385.
The progression of sondes with a relatively small section in the production zones presents no particular difficulty when the wells are substantially vertical and/or clear. It is thus conceivable to use light sondes which are less likely to have an effect on the frequency response of the measuring sensors which are included within. Another difficulty still remains which is linked to the permanent connection between the sonde and the electric-carrying cable. The latter, because of its great length, is subjected to pendular movements or acts as a transmittor of extraneous vibrations. The response of the sensors is distorted thereby and the signal to noise ratio is damaged.
French patent 2,564,599 assigned to the same assignee describes a measuring sonde whith sensors are arranged in a light basket hanging under a main unit comprising a hydraulic anchoring system by means of a supple connection and having thereby a mechanical uncoupling favouring the good reception of the signals to be picked up.
Using the sondes mentioned above, be it light sondes or a sonde with a suspended basket, is still sometimes impossible. This is the case when measurings are to be performed in zones where the progression is hindered either because the well is partly obstructed or because the sonde progresses in well portions that are more or less inclined in relation to the vertical. The sonde has to be loaded with added masses in order to overcome the increased frictional forces. This solution nevertheless has many drawbacks. The sensors arranged within the tool or sonde : geophones, accelerometers and/or possibly hydrophones, are actually mechanically coupled with a heavier tool body, which has the effect of damaging the signal to noise ratio of the signals they pick up.